I disagree, mostly because of how difficult it is (or in this case, is going to be) to draft and develop top pairing defensemen unless you really suck, and how trading for someone under 25 who is trending to being a top pairing D is nearly impossible because no one is going to want to let that player go, and likely doesn't have to as long as the cap keeps going up.
I see (theoretically) trading Buchnevich for Theodore as an immediate improvement of our D at the expense of our forward group, which has far more options to back fill, even if it means using free agency. I don't think you're going to get anything approaching the value equivalent of an under-25 D with Top 2 potential for Buch, and any draft picks we would get for him would be more like lottery tickets - no one is giving up a Top 5 pick, even if we throw in 16OA. The only possible exception I see would be prying one of Power or Byram out of Buffalo, but they're probably more interested in a younger forward or a RD compliment than they are for a guy like Buch who is one year from free agency.
Expecting to hit home runs to get back to true contention seems to me to be a fool's errand unless we somehow luck into a lottery win after missing the playoffs. The more likely path is 3-6 years of continuous improvement, and someone like Theodore would go a long way towards that. True, he will be aging out by the end of that time frame, but he will give you a number of years of significantly better play than what we have today. He would immediately be our Top LD (he may even be better than Parayko with his higher offensive upside) and allows Leddy to slot lower where he will almost certainly be more effective. The extension of Theodore would be pricey, but not that much more so than one for Buchnevich. Eventually, with the cap going up, you have an opportunity bring in someone in a couple of years that theoretically pushes Theodore (and/or Parayko) down in the lineup, either through free agency (which is likely going to be someone just a year or two younger than Theodore is now) or getting really lucky drafting a 1D outside the Top 16 and hoping he is ready by the time you plan to be contending.
Once Krug and Faulk are off the books, we will almost certainly have a ton of cap space available to improve the D, and bringing in someone like Theodore is not going to prevent us from building a Top 10 caliber defense. Quite the opposite, in my view, as I feel like his play style will keep him a legitimate 3/4 even at the end of a theoretical 6 year extension and we will get plenty of value out of him as we are trying to re-tool back to contender status.